1. Field of the Invention
This invention relates to stent slippage detection, and more specifically this invention relates to a system and method for determining, in situ, slippage of radioactive stents used to treat malignant and nonmalignant diseases.
2. Background of the Invention
Some cancers and neoplasms are easier to treat with radiation than others. Hard-to-reach neoplasms, such as those in the esophagus, intestines and other lumens, are often treated via Brachytherapy so as to minimize radiation to adjacent, healthy tissue.
Brachytherapy delivers radiation to small tissue volumes while limiting exposure of healthy tissue. In this regard, the delivered radiation conforms more to the target than any other form of radiation, (including proton therapy) as less normal, transient tissue is treated. It features placement of radiation sources, such as small radioactive particles or needles, near or within the target tissue, thus having the advantage over External Beam Radiation Therapy (EBRT) of being more focalized and less damaging to surrounding healthy tissue.
Brachytherapy is a common treatment for esophageal, prostate, and other cancers. Approximately 15,000 and 480,000 cases of esophageal cancer are diagnosed in the U.S. and worldwide, respectively. At least 50 percent of patients fail locally who present with curable cancers, which is to say that 50 percent suffer from persistence or recurrence of the cancers at the original cancer site. Another 50 percent of patients require dysphagia palliation.
Brachytherapy can be delivered in several rates: a High-Dose Rate (HDR), a Low-Dose Rate (LDR), and a very Low Dose Rate vLDR. The rates are expressed in Grays (Gy/hour) which are SI units of energy absorbed from ionizing radiation, equal to the absorption of one joule of radiation energy by one kilogram of matter.
LDR is an implant in which the tumor receives continuous radiation dose at about 8-12 Gy per day. Since the inception of brachytherapy at the beginning of the 20th century (i.e., soon after the discovery of radiation) delivery has been predominately LDR. Therefore, much of the long term data is LDR based.
Optimal dosimetry depends on the geometry of the radiation sources relative to the target to be treated. For example, if there is slippage distally down the GI tract, the delivered radiation would be suboptimal or compromised. This occurred during the phase two study of external beam radiation brachytherapy, and concurrent chemotherapy for patients with localized carcinoma of the esophagus (Radiation Therapy Oncology Group Study 9207 final report) where low dose radiation (LDR) seeds were also used as a comparison to HDR treatment. Gaspar L E, Qian C, Kocha W I, Coia L R, Herskovic A, Graham M A phase I/II study of external beam radiotherapy, brachytherapy and concurrent chemotherapy in localized cancer of the esophagus (RTOG 9207) preliminary toxicity report Int J Radiat Biol Phys 1997 Feb. 1:37 593-9. The freely moving radiation sources within the lumen led to bad implant dosimetries, thereby increasing the risk of being ineffective or causing injury.
Stent slippage has been reported approximately 15 percent of the time. Actual slippage rates are probably higher inasmuch as not all slippages are detected. Slippage may be due to peristalsis, gravity, tumor shrinkage, and opening of the obstruction. Generally, stent drift is in the superior-inferior direction, primarily in one plane, due to gravity. However, slippage in other directions can also occur. If the sources slip past the obstruction, suboptimal conditions could result. For example, if the tumor is 5 cm in its long axis, the radioactive part of a slipping implant is also 5 cm, and there is about 2 cm of inferior slippage, then the top half of the tumor would not be adequately irradiated while healthy parts of the esophagus inferior (i.e. downstream) from the tumor may receive too much radiation. Such instances result in what is known as radiation adverse reactions. Generally, slippage of more than two centimeters is considered counterproductive. For example, this slippage, in exposing healthy esophagus parenchyma, adds an extra margin of esophageal tissue which must now be monitored for radiation-induced anomalies or burns. Depending on factors associated with gastro intestinal stents, slippage of the stents within the GI tract can occur more often than stent sites outside of the GI tract. It is anticipated that since the goal of the radiation stent is to relieve obstruction mechanically and also by tumor shrinkage, stent slippage may occur more often. Slippage of prototypical non-radiation esophageal stents probably also occurs.
Stent slippage can occur especially if an obstructive lesion or tumor is dilated, as is often the objective. Brachytherapy dosimetries directly relate to outcome; therefore, an inexpensive methodology to raise suspicion of stent slippage would be helpful. If clinically appropriate, the physician would send the patient for confirming imaging.
In summary, it would be good medical practice that the clinician know of any increased likelihood of sub optimal radiation delivery due to stent slippage as soon as possible.
A need exists in the art for a system and method to detect slippage of medically applied stents, in situ. The system and method should utilize ordinary materials and radiation detection equipment. Such an optimized system and method, as described herein, could be performed at a physician's office or other outpatient scenario, within a matter of 15-20 minutes and be able to detect slippage distances of as little as about 10 millimeters (mm). Alternatively, such a system and method, as described herein, enables the patient to detect stent slippage between doctor visits by detecting radioactive emissions that slip from underneath a radio-opaque shield, or that emanate through apertures in the shield heretofore not experiencing emanations.